Calibration methods for gas chromatograph-mass spectrometers (GC-MS) typically employ a fluorinated calibrant. This calibrant is generally used both as a mass calibrant to properly assign ion masses, and as a resolution calibrant to adjust for proper mass peak widths when tuning quadrupole based mass spectrometers. The fluorinated compound selected may include high or low boiling perfluorokerosene, perfluorotributylamine (FC-43), perfluorotripentylamine (FC-70) or similar calibrants. Perfluorinated mass calibrants are generally preferred due to the monoisotopic nature of fluorine, the negative mass defect of resulting ions, and the ability of these classes of mixtures and compounds to exist as a liquid at room temperature while also having a high molecular weight. This allows a simple vapor based introduction of a high mass calibrant from a headspace of a calibration vial at ambient temperature without the need for heating the calibrant vial and associated delivery hardware.
While suitable for general electron ionization (EI) and positive chemical ionization (PCI) over a wide mass range, the above calibrants suffer in negative ion chemical ionization (NCI) due to low ion abundance below about mass to charge ratio (m/z) 150 daltons (Da). Low ion abundances in this region are due to the very low energy transfer of the NCI ionization process which precludes significant fragmentation of parent molecules. This presents problems when accurate mass calibration or resolution calibration is needed at lower masses, as accurate mass axis calibration and peak resolution calibration are very difficult if not impossible with poor ion abundances.
A calibrant such as perfluorotributylamine may contain a mixture of positional isomers due to perfluorinated n-butyl, sec-butyl, iso-butyl and tert-butyl groups. Even these isomers have very similar vapor pressures, identical molecular mass and quite similar fragmentation patterns. Identical mass and similar vapor pressure and fragmentation pathway allows long term usage without observable change over time in mass spectral peak abundances. Peak abundances which do not change over time is considered an advantage for repeatable instrument calibration as well as customer perception. The disadvantage of using isomeric mixtures or single compounds for mass calibration is that ion diversity is limited by the fragmentation pathway of a single molecular weight. For NCI operation wherein fragmentation is largely limited, a single molecular weight calibrant limits ion diversity and low mass ion intensity even further. By contrast, calibrant mixtures are known such as high and low molecular weight perfluorokerosene which contain a range of fluorocarbons having similar chemistry but differing mass. This offers the ability for an extended range of mass calibration for NCI (particularly at high mass), but low mass ion intensity of these mixtures is generally low due to the low mole fraction of low molecular weight fluorocarbons. Mixtures of these types can also suffer from long term shifts in ion abundances due the changing headspace concentration of differing molecular weight species.
Traditional methods of introducing calibration compounds, e.g., perfluorotributylamine to the ionization region of mass spectrometers often involve intermediate use of ball valves or needle valves or similar gate devices between a source of calibrant vapor and an ion source of a mass spectrometer. Alternatively, calibrants may be delivered indirectly to the ion source of the mass spectrometer by flooding the vacuum chamber with a pre-evacuated vial containing such compounds. Instrument designs which feed the calibrant directly to the ion source of the mass spectrometer typically utilize needle valves due to the extremely small quantities (<4 ng/s) of calibrant required for effective instrument calibration. These valves may be expensive due to close machining tolerances involved in their production.
Different quantities of calibrants may be necessary based on whether an instrument is operating in electron ionization (EI), positive ion chemical ionization (PCI), or negative ion chemical ionization (NCI) modes of operation. Other difficulties arising from such metering methods may include: poor regulation due to variations in headspace pressure, poor regulation due to self-contamination (for example, from outgas sing of adsorbed calibrant from O-rings, valve seats, ceramics and internal packings), lack of reproducibility when returning a valve to a previous setting, inaccurate or unknown volume delivery and poor equilibrium time.
U.S. Pat. No. 7,737,395 describes a mass calibration formulation using a mass calibrant along with a “moderator substance” having a lower vapor pressure than the mass calibrant. The mass calibrant (0.1%-10% composition in the mixture) may comprise FC-70 or other low vapor pressure chemistries suitable for damping the intensity of the calibrant ions. The moderator may also serve to generate calibration ions and may include common mass spectrometry calibrants such as, fluorinated polyphenylethers, polyfluoroalkyls, polysilicones, triperfluoroalkylamines, etc. A potential feature of the above methodology is that high vapor pressure fluids (generally lower molecular weight) which might yield intense lower calibration masses can be dampened in accordance with their concentration in the moderator. The vapor pressure of any given compound in an ideal mixture can be expressed in accordance with Raoult's law. Raoult's law states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture. Calibrants which approach the theory of Raoult's law may be expected to have similar chemistries e.g. FC-43 and FC-70 which are both triperfluoroalkylamines.
Mass calibrants have also been introduced specifically for chemical ionization (CI) operation which can extend to lower masses. One such material is PFDTD (perfluorodimethyltrioxadodecane). PFDTD can offer advantages for negative ion mass calibration as it covers a lower mass range compared to FC-43. The minimum useful mass for FC-43 in NCI mode is generally m/z 283 Da. Even so, it represents only about 5% relative abundance compared to the next higher mass of m/z 452. By contrast, PFDTD offers intense ions at m/z 185, 351 and 449 for methane NCI. While this calibrant extends calibration to lower masses relative to FC-43, even lower mass calibration is desirable.
The use of PFDTD generally is reserved for CI calibration (PCI and NCI inclusive) using additional metering hardware. The need for additional hardware arises from the fact that CI operation is carried out at elevated ion source pressures in order to favor ion-molecule reactions between reagent ions and neutral analyte molecules. This requires metering the calibrant directly into the ion source rather than delivering it into the vacuum manifold as is often done with FC-43 during EI calibration. This additional hardware results in higher manufacturing costs.
A significant drawback of the mixture approach as described in U.S. Pat. No. 7,737,395 is the inherent time varying intensity of each calibrant represented in the vial headspace. The concentrations of each calibrant will vary over time unless the evaporation of the mixture constitutes the behavior of an azeotrope. In addition, it is unlikely that a real mixture of calibrants would adhere to Raoult's law, which would be necessary if the mixture were to lend itself to a predictable change in concentration over time. While mass spectrometers can calibrate over a wide range in ion intensities, it is more desirable to have a consistent mass spectrum which remains unchanged over the course of many weeks than it is to have mass peaks which initially have a given intensity and then change over time. Such changes could imply for example a mass-dependent roll-off in instrument response rather than a change in composition of the calibrant.
U.S. Pat. No. 6,635,885 describes an apparatus designed to provide a continuously regenerated quasi-equilibrated calibrant vapor to the ion source of a mass spectrometer. The design of this invention is such that the calibration gas is always in an “on” state, being delivered either to a mass spectrometer ion source or to a roughing vacuum pump. This allows the vapor to be in a state of perpetual equilibrium with surfaces at all times such that a rapid and stable response is achieved when a calibration source is required for EI or CI modes of operation. Very low concentrations of vapors in the gas phase can be subject to adsorption and desorption effects on active surfaces of plumbing and valve components resulting in poor reproducibility and response times. U.S. Pat. No. 6,635,885 largely eliminates these ill effects. Against the above background there is need for an improved multi-calibrant, time-stable, extended range mass calibration apparatus and method.